Dragonfly blue-tailed damselfly (Ischnura elegans)
Wikipedia: Ischnura is a kind of insects zygoptères Odonata (damselflies) the family Coenagrionidae.
French vernacular names: the Ischnures or Agrions (Agrions also designating other genres).

photo taken in the nature reserve:
The pond of Royes is a pond that is located in the Freiberger (Canton of Jura), on the territory of the commune of Saignelégie